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Abstract. Large subduction earthquakes can rupture the shallow part of the megathrust with unusually large displacements and tsunamis. The long duration of the seismic source and high upper-plate compliance contribute to large and protracted long-period motions of the outer upper plate. The resulting shear stress at the sediment–water interface in, for example, the Mw 9.0 2011 Tohoku–Oki earthquake could account for surficial sediment remobilization on the outer margin. We test this hypothesis by simulating in physical tank experiments the combined effects of high- and low-frequency seismic motions on sediment of different properties (chemistry, grain size, water content, and salinity). Our results show that low-frequency motion during a 2011-like earthquake can entrain several centimeters of surficial sediment and that entrainment can be enhanced by high-frequency vertical oscillations. These experiments validate a new mechanism of co-seismic sediment entrainment in deep-water environments. 

Submarine paleoseismology hinges on analyzing stratigraphic records to learn about past earthquakes. Marine microfossils, such as foraminifera, can reveal critical details about the water depth of the sediment remobilized during earthquakes.Although allochthonous foraminifera are typically excluded from age control studies, these faunas are useful in providing insight into the water depth from which a sediment transport deposit originated. As an example, we describe a ~16 m thick Holocene sedimentary deposit first defined by Kioka et al. (2019) and later dated to 1.77 (+0.49/−0.31) ka by Usami et al. (2021) and Schwestermann et al. (2021) based on bulk OC, C-14. Strasser et al. (InPress) identified the event at IODP Expedition 386 Sites M0084, M0086, and M0088, located at hadal depths in the northern Japan Trench. Each site features a basal 1 to2 m thick, fining-upward medium sand to silt with well-defined planar and ripple lamination. The sequence has been interpreted as a turbidite composed of basalsand grading into silty clay that is overlain by a mass transport deposit. Calcareous foraminifera occur in the deposit despite being well below the CCD. Foraminiferal abundance decreases from the basal sand to the fi ne silty clay due to gravitational size sorting. Most displaced foraminifera, including thin-shelled taxa, are moderatelyt o well-preserved, likely due rapid burial, high alkalinity, and low internal friction within the flow. The foraminiferal assemblages are consistent across the three sites. The sandy portion of the deposit is dominated by Elphidium batialis, Uvigerina akitaensis, Nonionellina labradorica, Chilostomellina fi mbriata, and species of Bulimina, indicative of upper to middle bathyal depths (200 to 1000 m). In contrast, the silty clay contains smaller foraminifera like Bolivina, Cassidulina, Stainforthia and Epistominella, suggesting outer neritic to upper bathyal depths (100 to 600 m). Radiocarbon dating of the foraminifers within the basal sand from each site reveals that M0086 (16750 to 16200 cal BP) and M0088 (16650 to 16050 cal BP) have younger source ages compared to M0084 (19350 to 18800 cal BP). These findings indicate that the ~1.77 ka sediment transport event entrained much older strata that had originated in upper to middle bathyal depths and outer neritic to upper bathyal depths. 

The largest known earthquakes ruptured megathrusts at subduction boundaries. The largest among these ruptured the entire seismogenic depth range up to the seafl oor and have generated enormous regionally destructive tsunamis. This type of rupture that breaches the sea-fl oor is fortunately rare, but, as a result, the most recent ones, M9.2 Sumatra in 2004 and the M9.0 Japan in 2011, were unexpected and thus caused great damage. We don’t know where and when they can occur again. Our approach has been to compare earthquake event deposits in various ocean settings (IODP Expedition 386, Japan 2021; Jamaica Passage 2022; Bay of Bengal 2024) and to study the entrainment processes (shaking tank experiments) and search for distinguishing features in the depositional record. We are now revealing techniques that involve the use of isotopes and chemistry to characterize earthquake related event deposits. We identifi ed thick, acoustically homogeneous layers “homogenites” that have homogeneous radiogenic isotope (Nd, Sr, Pb) signatures, unlike the background sediments. Additionally, TOC%, N% and d C, d N, show distinct signatures relative to the background. These isotopic signatures correspond perfectly well with lithology, physical properties and X-CT scans in the thick homogenites. Using these techniques we recognize the 1454 AD Kiatoka and 869 AD Jogan events in the Japan Trench that were tsunamigenic and possibly ruptured the seafl oor. While each of these events has unique signatures, there are common threads and these fi ndings lay the groundwork to go back in time and better characterize older Mw9.0 ruptures. One of the most signifi cant contributions to this effort is the recognition of M9.0 2011 Tohoku tsunamigenic earthquake in the Japan Trench. Short-lived radioisotopes help to document the extent of the remobilized sediment. This event has provided unique insights due to the Fukushima nuclear reactor radioisotopes measured in the Japan Trench as far as ~200km from its source. The use of these techniques provides tools for recognizing tsunamigenic earthquakes in other subduction boundaries such as Cascadia. 

The 2011 Mw9.0 Tohoku-Oki earthquake may be representative of “maximum”earthquakes: it ruptured the entire seismogenic depth range of the Japan megathrust, including the shallowest segment that reaches the trench where the displacement grew to 60 m and spawned a catastrophic tsunami. Models and direct seafloor measurements imply a comparably large initial relative motion and sustained long-period oscillations between sediment and water at the seafloor above the shallowest megathrust segment. This motion may develop enough shear to re-suspend sediment, but exclusively for the maximum earthquakes. This new co-seismic sediment-entrainment process should leave a recognizable sedimentary fingerprint of these earthquakes. Our physical experiments are testing effects of this shear between sediment and water and its interaction with high-frequency vertical shaking. We also investigate the impact of sediment properties and slope on the entrainment. We worked on several synthetic mixtures, defined according to the grain size distribution, clay mineralogy and water content with either freshwater or sea water. The grain size distribution is simplified but matches those of sediment cores from different subduction zones. For each mixture, we built matrices of the erosion rates according to the flow velocities, which shows the role of water content and vertical shaking. We have also identifi ed different mechanism during the runs:grain-by-grain or clasts entrainment, stripping, motion of the sediment interface, and formation of a dense sediment layer above the surface. These observations maybe recorded in the associated deposit, suggesting different fingerprinting by the tsunamigenic earthquakes depending on the characteristics of each subduction zone. 

International Ocean Discovery Program (IODP) Expedition 386, Japan Trench Paleoseismology (offshore period: 13 April to 1 June 2021; Onshore Science Party: 14 February to 14 March 2022) was designed to test the concept of submarine paleoseismology in the Japan Trench, the area where the last, and globally only one out of four instrumentally-recorded, giant (i.e. magnitude 9 class) earthquake occurred back in 2011. “Submarine paleoseismology” is a promising approach to investigate deposits from the deep sea, where earthquakes leave traces preserved in the stratigraphic succession, to reconstruct the long-term history of earthquakes and to deliver observational data that help to reduce uncertainties in seismic hazard assessment for long return periods. This expedition marks the first time, giant piston coring (GPC) was used in IODP, and also the first time, partner IODP implementing organizations cooperated in jointly implementing a mission-specific platform expedition. We successfully collected 29 GPCs at 15 sites (1 to 3 holes each; total core recovery 831 meters), recovering 20 to 40-meter-long, continuous, upper Pleistocene to Holocene stratigraphic successions of 11 individual trench-fill basins along an axis-parallel transect from 36°N – 40.4°N, at water depth between 7445-8023 m below sea level. These offshore expedition achievements reveal the first high-temporal and high spatial resolution investigation and sampling of a hadal oceanic trench, that form the deepest and least explored environments on our planet. The cores are currently being examined by multimethod applications to characterize and date hadal trench sediments and extreme event deposits, for which the detailed sedimentological, physical and (bio-)geochemical features, stratigraphic expressions and spatiotemporal distribution will be analyzed for proxy evidence of giant earthquakes and (bio-)geochemical cycling in deep sea sediments. Initial preliminary results presented in this EGU presentation reveal event-stratigraphic successions comprising several 10s of potentially giant-earthquake related event beds, revealing a fascinating record that will unravel the earthquake history of the different along-strike segments that is 10–100 times longer than currently available information. Post-Expedition research projects further analyzing these initial IODP data sets will (i) enable statistically robust assessment of the recurrence patterns of giant earthquakes, there while advancing our understanding of earthquake induced geohazards along subduction zones and (ii) provide new constraints on sediment and carbon flux of event-triggered sediment mobilization to a deep-sea trench and its influence on the hadal environment. IODP Expedition 386 Science Party: Piero Bellanova; Morgane Brunet; Zhirong Cai; Antonio Cattaneo; Tae Soo Chang; Kanhsi Hsiung; Takashi Ishizawa; Takuya Itaki; Kana Jitsuno; Joel Johnson; Toshiya Kanamatsu; Myra Keep; Arata Kioka; Christian Maerz; Cecilia McHugh; Aaron Micallef; Luo Min; Dhananjai Pandey; Jean Noel Proust; Troy Rasbury; Natascha Riedinger; Rui Bao; Yasufumi Satoguchi; Derek Sawyer; Chloe Seibert; Maxwell Silver; Susanne Straub; Joonas Virtasalo; Yonghong Wang; Ting-Wei Wu; Sarah Zellers 

Short historical and even shorter instrumental records limit our perspective of earthquake maximum magnitude and recurrence, and thus are inadequate to fully characterize Earth’s complex and multiscale seismic behavior and its consequences. Examining prehistoric events preserved in the geological record is essential to reconstruct the long-term history of earthquakes and to deliver observational data that help to reduce uncertainties in seismic hazard assessment for long return periods. Motivated by the mission to fill the gap in long-term records of giant (Mw 9 class) earthquakes such as the Tohoku-Oki earthquake in 2011, International Ocean Discovery Program (IODP) Expedition 386, Japan Trench Paleoseismology, was designed to test and further develop submarine paleoseismology in the Japan Trench. Earthquake rupture propagation to the trench and sediment remobilization related to the 2011 Mw 9.0 Tohoku-Oki earthquake, and the respective structures and deposits are preserved in trench basins formed by flexural bending of the subducting Pacific Plate. These basins are ideal study areas for testing event deposits for earthquake triggering as they have poorly connected sediment transport pathways from the shelf and experience high sedimentation rates and low benthos activity (and thus high preservation potential) in the ultra-deep water hadal environment. Results from conventional coring covering the last ~1,500 y reveal good agreement between the sedimentary record and historical documents. Subbottom profile data are consistent with basin-fill successions of episodic muddy turbidite deposition and thus define clear targets for paleoseismologic investigations on longer timescales accessible only by deeper coring. In 2021, IODP Expedition 386 successfully collected 29 Giant Piston cores at 15 sites (1 to 3 holes each; total core recovery 831 meters), recovering 20 to 40-meter-long, continuous, upper Pleistocene to Holocene stratigraphic successions of 11 individual trench-fill basins along an axis parallel transect from 36°N – 40.4°N, at water depth between 7445-8023 m below sea level. The cores are currently being examined by multi-method applications to characterize and date event deposits for which the detailed stratigraphic expressions and spatiotemporal distribution will be analyzed for proxy evidence of giant versus smaller earthquakes versus other driving mechanisms. Initial preliminary results presented in this EGU presentation reveal event-stratigraphic successions comprising several 10s of potentially giant-earthquake related event beds, revealing a fascinating record that will unravel the earthquake history of the different along-strike segments, that is 10–100 times longer than currently available information. The data set will enable a statistically robust assessment of the recurrence patterns of giant earthquakes as input for improved probabilistic seismic hazard assessment and advanced understanding of earthquake induced geohazards globally. IODP Expedition 386 Science Party: Piero Bellanova; Morgane Brunet; Zhirong Cai; Antonio Cattaneo; Tae Soo Chang; Kanhsi Hsiung; Takashi Ishizawa; Takuya Itaki; Kana Jitsuno; Joel Johnson; Toshiya Kanamatsu; Myra Keep; Arata Kioka; Christian Maerz; Cecilia McHugh; Aaron Micallef; Luo Min; Dhananjai Pandey; Jean Noel Proust; Troy Rasbury; Natascha Riedinger; Rui Bao; Yasufumi Satoguchi; Derek Sawyer; Chloe Seibert; Maxwell Silver; Susanne Straub; Joonas Virtasalo; Yonghong Wang; Ting-Wei Wu; Sarah Zellers